As is known, hose-and-drogue in-flight refueling systems comprise: a tanker aircraft equipped with a refueling hose fitted on its free end with a drogue; and a fuel take-on aircraft equipped with a rigid probe designed to connect to the drogue to allow the tanker aircraft to refuel the take-on aircraft.
The procedure adopted by conventional hose-and-drogue refueling systems substantially comprises: a tanker aircraft positioning step, in which the tanker aircraft follows a predetermined course, normally a straight line, at a constant speed of 200 to 350 knots, depending on the aircraft involved, so as to move as predictably as possible; an approach or docking step, in which the pilot of the take-on aircraft moves the aircraft into a receiving zone to dock with the tanker aircraft; a drogue-probe coupling step, in which the pilot of the take-on aircraft controls the aircraft manually to align the probe with the drogue and move the aircraft into position to connect the probe to the drogue; a refueling step, in which fuel is transferred from the tanker aircraft to the take-on aircraft via the refueling circuit, in particular the drogue-probe; and a drogue-probe release step.
The above procedure involves numerous technical problems.
Firstly, successful drogue-probe coupling, and the time taken to do it, depend on various factors, such as: weather conditions; visibility; and the stress level and skill of the pilots of the take-on aircraft in performing the manual docking/coupling manoeuvre.
Secondly, drogue-probe coupling is only successful when the relative speed of the two aircraft, over the last few seconds prior to coupling, ranges between 2 and 5 knots. In fact, below 2 knots, the probe cannot be connected to the drogue; whereas speeds of over 5 knots may result in hazardous docking of the tanker and take-on aircraft.
In refueling systems of the type described, drogue-probe coupling is therefore a critical operation which, in addition to the failure margins involved, is a source of severe stress on the pilots performing the manoeuvre, and does not allow for calculating refueling time, which obviously poses serious problems when a whole fleet has to be refueled within a maximum total time associated with the fuel range of the aircraft in the fleet.